The invention discloses a cylinder liner which is sealed into a reciprocating piston engine, comprising a supereutectic aluminum/silicon alloy and a method of producing such a cylinder liner, in which the surface of the cylinder is first roughly machined, then fine machined by boring or turning, and subsequently honed in at least one stage, in which the surface particles which are harder than the base microstructure of the alloy, such as silicon crystals and/or intermetallic phases, are then exposed in level areas projecting above the remaining surface of the base microstructure of the alloy.
Hagiwara, et al., EP 367,229 discloses a cylinder liner which is made of metal powder, such as aluminum oxide, with from 0.5 to 3% graphite particles mixed-in, which have a particle diameter of at most 10 .mu.m or less (measured in a plane perpendicular to the cylinder axis) and from 3 to 5% hard material particles without sharp edges, which have a particle diameter of at most 30 .mu.m and on an average 10 .mu.m or less. The metal powder is produced first, without mixing-in the nonmetallic particles, by air atomization of a supereutectic aluminum/silicon alloy having the following composition, with the remainder being aluminum (figures are in % weight based on the total metal content of the alloy, i.e. without the particles of hard material and graphite):
Silicon from 16 to 18 g, PA1 Iron from 4 to 6%, PA1 Copper from 2 to 4%, PA1 Magnesium from 0.5 to 2% and PA1 Manganese from 0.1 to 0.8% PA1 Silicon: 22.8%, PA1 Copper: 3.1%, PA1 Magnesium: 1.3%, PA1 Iron: 0.5% and PA1 Nickel: 8.0%. PA1 Silicon: 25%, PA1 Copper: 4.3%, PA1 Magnesium: 0.65% and PA1 Iron: 0.8%.
The metal powder is mixed with the nonmetallic particles and then pressed at about 2000 bar to make a preferably tubular body. This powder metallurgically produced blank is inserted into a soft aluminum tube of corresponding shape to make a double layer tube, which is jointly sintered and shaped in an extrusion processing preferably at elevated temperatures, to give a tubular blank from which the individual cylinder liners can be produced.
The embedded particles of hard material are intended to give the cylinder liner good wear resistance, while the graphite particles serve as dry lubricants. However, to avoid oxidation of the graphite particles, the hot extrusion should take place in the absence of oxygen. There is also the danger of the graphite reacting with the silicon at high processing temperatures and forming hard SiC on the surface, which interferes with the dry-lubricating properties of the embedded graphite particles. Furthermore, local surface fluctuations in the concentration of particles of hard material and/or graphite can never be entirely eliminated.
Other disadvantages of Hagiwara, et al. '229, are due to the fact that the embedded particles of hard material, despite their rounded edges, still have strong abrasive action, thereby causing the hotpressing die to wear out relatively quickly. In any case, only a partial rounding of the particle edges formed by crushing can be achieved with justifiable effort. High tool wear, and thus high tool costs, is also associated with the subsequent machining of the surface of the cylinder liner. After machining, the hard material particles exposed on the surface, have sharp edges and cause relatively high wear of the piston and the piston rings, therefore, these have to be made of wear-resistant material or be provided with appropriate wear resistant coating.
Basically, the Hagiwara, et al. '229 cylinder liner is not only relatively expensive because the starting materials require several separate components, but also because of the high tool costs associated with the process. Additionally, because these known cylinder liners are produced from a heterogeneous powder mixture, the danger of inhomogeneities exists, which may result in impaired function, and thus in rejects, requiring careful quality control. In addition, for use in an engine, complicated piston construction is required, which makes the entire reciprocating piston engine more expensive.
Kiyota, et al., U.S. Pat. No. 4,938,810, likewise discloses a powder-metallurgically produced cylinder liner. In Kiyota, et al. '810, the silicon content of the examples provided are in the range of from 10 to 30%, which extends into the subeutectic region, and preferably from 17.2 to 23.6%. At least one of the metals nickel, iron, or manganese, should be present in the alloy to the extent of at least 5%, or in the case of iron, to the extent of at least 3%. To follow is an example in Kiyota, et al. '810 of an alloy composition in % by weight, where the remainder is aluminum, and the content of zinc and manganese are not specified, and are therefore assumed to be present in trace quantities only:
It should be noted that the nickel content in the alloy example given above is very high- Kiyota, et al. '810 further discloses that a blank for a cylinder liner is hot-extruded from the powder mixture.
Perrot, et al., U.S. Pat. No. 4,155,756, also concerns a powder-metallurgically produced cylinder liner. In one example, the composition is as follows, with the remainder being aluminum:
An object of the present invention is to improve cylinder liners by increasing wear resistance, thereby reducing the danger of wear on the piston, and decreasing the amount of lubricating oil necessary. The main interest in reducing the amount of lubricating oil necessary does not so much concern the lubricating oil itself, but rather its combustion residues, essentially hydrocarbons, which pollute the exhaust gas emitted from internal combustion engines.
This object is achieved according to the present invention by a cylinder liner which is sealed into a reciprocating piston engine, comprising a supereutectic aluminum/silicon alloy and a method of producing such a cylinder liner, in which the surface of the cylinder is first roughly machined, then fine machined by boring or turning, and subsequently honed in at least one stage. As a result, the surface particles which are harder than the base microstructure of the alloy, such as silicon crystals and/or intermetallic phases, are exposed in level areas projecting above the remaining surface of the base microstructure of the alloy.
The specific alloy composition of the material used for the cylinder liner allows silicon primary crystals and intermetallic phases to be formed directly from the melt, therefore, there is no need to separately mix-in hard particles. Furthermore, spray compaction of the alloy, a known process which can be readily mastered and is comparatively inexpensive, is used together with subsequent, energy-saving cold extrusion of the blank. This method results in particularly low oxidation of the droplet surfaces and particularly low porosity of the liner. The alloy compositions A and B mentioned below are for use respectively with iron-coated pistons and with uncoated aluminum pistons.
The hard particles formed from the melt have a high hardness and give the surface good wear resistance without seriously impeding the machining of the material, so that the surface is sufficiently readily machinable. Furthermore, because of the formation of the primary crystals and intermetallic phases in each melt droplet sprayed onto and subsequently solidifying on the blank, the process results in a very uniform distribution of hard particles on the workpiece. The particles formed from the melt are also less angular and tribologically less aggressive than crushed particles. Moreover, hard metallic particles formed from the melt are more intimately embedded in the basic alloy microstructure than are nonmetallic crushed particles which have been mixed in. This factor lowers the danger of crack formation at the boundaries of the hard particles. In addition, the hard particles formed from the melt display better breaking-in behavior and lower abrasive aggressivity towards the piston and its rings, so that longer lifetimes result or, in any case, so that less complex piston designs are possible.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.